The national gas infrastructure of the United States is both vast and varied. Materials used in the construction of pipeline, their age and their location are major variables in maintaining pipeline integrity. The ability to inexpensively and efficiently monitor and assess pipeline integrity and status may provide improved means for service-life prediction and defect detection to ensure operational reliability. Existing techniques do not work well, and the expense for using eddy currents, as an example, is approximately $1 M per 100 mile of pipe, and can only detect gross defects in the pipes. Visual inspection using cameras provides little information concerning the integrity of the pipeline.
The most common causes of pipeline failure in North America include mechanical damage; that is, denting or gouging of the pipeline caused by workers (digging using backhoes, as an example), and natural corrosion over time. In many cases, the pipes will fail under load unless defects are detected in a timely manner. In other situations, they may remain undetected, with the local damage acting as sites for further corrosion or cracking, and potentially leading to a delayed failure, such as an explosion.
Presently, visual inspections employing video cameras are the primary means for inspection of the interiors of natural gas pipelines. However, such procedures do not permit one to view damage to the outside surface of pipe. Eddy currents generated by strong magnets placed close to the inner surface of the pipe are also used to detect features. However, since natural gas pipelines are made of steel, moving such magnets through the pipe may be difficult due to the Eddy current braking effect. Additionally, because of the close proximity of the magnets to the interior of the pipe, the sensor elements may scrape the inner wall of the pipe, thereby fouling the sensor. The interiors of the pipes must therefore be cleaned with metal brushes before this procedure is utilized. The Magnetic flux leakage (MFL) technique suffers from the same difficulties as the eddy current method.
The United States has over 2 million miles of gas pipelines. Of interest is a sensing system that can be mounted on a ‘pig’ (a device inserted into a pipeline for inspection or cleaning purposes) which travels through the inside of a natural gas pipeline and is suitable for detection of wall defects such as corrosion pits on both the inside and the outside of pipe. It is of importance that the sensing system does not have rotating or otherwise moving parts in order to simplify the design, make it easier to maintain and also to conserve battery power for longer inspections.
In U.S. Pat. No. 6,186,004 for “Apparatus And Method For Remote, Noninvasive Characterization Of Structures And Fluids Inside Containers” which issued to Gregory Kaduchack et al. on Feb. 13, 2001, an apparatus and method for remote, non-contact evaluation of structures and containers at large distances (on the order of several meters) in air is described. The invention utilizes an air-coupled, parametric acoustic array to excite resonance vibrations of elastic, fluid-filled vessels and structural members, where a nonlinear mixing process in the air medium transforms highly directional, narrow beamwidth higher acoustic frequencies into lower acoustic frequencies suitable for vibrational excitation of common structures. Vibrations were readily detected using a laser vibrometer in a fixed position relative to the acoustic array. Interior fluid characterization was achieved by analyzing the propagation of the generated guided waves (for example, the lowest-order generalized antisymmetric Lamb wave, a0) which is guided by the circumference of the container. The a0 Lamb wave is in a class of guided waves which exhibit strong flexural vibrations near the resonance frequency of the container. It should be pointed out that the parametric array requires a minimum distance from the acoustic source of several wavelengths in air before it can generate the lower frequency sound wave, and thus cannot be fitted inside of typical natural gas pipelines that can range from 4 in. to 18 in. in diameter. Moreover, the mixing process is intended to produce frequencies less than about 40 kHz.
Pulse-echo, time-of-flight procedures have been used to determine sound propagation through materials. A narrow electrical pulse is used to excite a transducer which generates sound waves in an object such as a plate. The pulse propagates through the object and is detected by either the same transducer or by another. By determining the travel time over a known distance within the object, the sound velocity may be determined. A narrow pulse has high-frequency content requiring a high-bandwidth amplifier to detect the signals from the receiving transducer. Unfortunately, this exposes the measurement to the entire bandwidth of the amplifier. Further, typical pulse-echo measurements require transducer excitation voltages between 300 V and 500 V and much signal averaging or fast frequency chirp correlation techniques to make meaningful, air-coupled signal measurements.